U.S. patent number 6,947,788 [Application Number 09/879,109] was granted by the patent office on 2005-09-20 for navigable catheter.
This patent grant is currently assigned to Super Dimension Ltd.. Invention is credited to Danny Blecher, Pinhas Gilboa.
United States Patent |
6,947,788 |
Gilboa , et al. |
September 20, 2005 |
**Please see images for:
( Certificate of Correction ) ** |
Navigable catheter
Abstract
A catheter, including: a housing having a transverse inner
dimension of at most about two millimeters; and a coil arrangement
including five coils and five solid cores. Each of the coils is
wound around one of the solid cores. The coils are non-coaxial. The
coil arrangement is mounted inside the housing.
Inventors: |
Gilboa; Pinhas (Haifa,
IL), Blecher; Danny (Ramat Gan, IL) |
Assignee: |
Super Dimension Ltd. (Herzelia,
IL)
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Family
ID: |
26323687 |
Appl.
No.: |
09/879,109 |
Filed: |
June 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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463177 |
Jan 21, 2000 |
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Foreign Application Priority Data
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Aug 2, 1998 [IL] |
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125626 |
Oct 29, 1998 [IL] |
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126814 |
Jul 7, 1999 [WO] |
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PCT/IL99/00371 |
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Current U.S.
Class: |
600/435; 600/407;
600/433; 600/437; 600/459; 600/466 |
Current CPC
Class: |
A61B
5/06 (20130101); A61B 34/20 (20160201); A61B
5/062 (20130101); A61B 5/055 (20130101); A61B
6/032 (20130101); A61B 8/08 (20130101); A61B
8/4245 (20130101); A61B 2034/2072 (20160201); A61B
2034/2051 (20160201) |
Current International
Class: |
A61B
5/06 (20060101); A61B 19/00 (20060101); A61B
5/055 (20060101); A61B 6/03 (20060101); A61B
8/08 (20060101); A61B 006/00 () |
Field of
Search: |
;600/435,407,433,437,459,466 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO95/09562 |
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Apr 1995 |
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WO |
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WO96/05768 |
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Feb 1996 |
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WO |
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WO97/36143 |
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Oct 1997 |
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WO |
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Primary Examiner: Mantis-Mercader; Eleni
Assistant Examiner: Jung; William
Attorney, Agent or Firm: Friedman; Mark M.
Parent Case Text
This is a Divisional of U.S. Ser. No. 09/463,177, filed Jun. 21,
2000.
Claims
What is claimed is:
1. A catheter, comprising: (a) a housing having a transverse inner
dimension of at most about two millimeters; and (b) a coil
arrangement including at least five coils and at least five solid
cores, each of said coils being wound around one of said cores,
said coils being non-coaxial, said coil arrangement being mounted
inside said housing.
2. The catheter of claim 1, wherein said solid cores includes
ferrite.
3. The catheter of claim 1, wherein said coils have an axis, said
axes of at least three of said coils being substantially mutually
perpendicular to each other.
4. The catheter of claim 1, wherein each of said coils has a
center, said centers of said coils lying on a substantially
straight line.
5. A probe for interacting with a body cavity, comprising: (a) a
substantially cylindrical catheter; (b) a satellite; being part of
a sensing probe; (c) a flexible pocket having an aperture therein,
said flexible being rigidly, attached to said catheter, said
flexible pocket being configured for receiving said satellite
therein; and (d) a tether passing through said aperture, said
tether being attached to said satellite, such that, after said
catheter and said satellite have been inserted into the body
cavity, said tether is withdrawn, thereby pulling said satellite
into said flexible pocket so as to reversibly secure said satellite
at a fixed position and orientation relative to said catheter.
6. The probe of claim 5, wherein said mechanism includes: (i) a
sleeve, rigidly secured to said tether and adapted to slide along
said catheter.
7. The probe of claim 5, wherein said sensor is configured for
sensing a position and orientation of said catheter.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to electromagnetic tracking devices
and, more particularly, to a system and method for tracking a
medical probe such as a catheter as the probe is moved through the
body of a patient.
It is known to track the position and orientation of a moving
object with respect to a fixed frame of reference, by equipping the
moving object with a transmitter that transmits electromagnetic
radiation, placing a receiver in a known and fixed position in the
fixed frame of reference, and inferring the continuously changing
position and orientation of the object from signals transmitted by
the transmitter and received by the receiver. Equivalently, by the
principle of reciprocity, the moving object is equipped with a
receiver, and a transmitter is placed in a known and fixed position
in the fixed frame of reference. Typically, the transmitter
includes three orthogonal magnetic dipole transmitting antennas;
the receiver includes three orthogonal magnetic dipole receiving
sensors; and the object is close enough to the stationary apparatus
(transmitter or receiver), and the frequencies of the signals are
sufficiently low, that the signals are near field signals. Also
typically, the system used is a closed loop system: the receiver is
hardwired to, and explicitly synchronized with, the transmitter.
Representative prior art patents in this field include U.S. Pat.
No. 4,287,809 and U.S. Pat. No. 4,394,831, to Egli et al.; U.S.
Pat. No. 4,737,794, to Jones; U.S. Pat. No. 4,742,356, to Kuipers;
U.S. Pat. No. 4,849,692, to Blood; and U.S. Pat. No. 5,347,289, to
Elhardt. Several of the prior art patents, notably Jones, present
non-iterative algorithms for computing the position and orientation
of magnetic dipole transmitters with respect to magnetic dipole
receivers.
An important variant of such systems is described in U.S. Pat. No.
5,600,330, to Blood. In Blood's system, the transmitter is fixed in
the fixed reference frame, and the receiver is attached to the
moving object. Blood's transmitting antennas are spatially
extended, and so cannot be treated as point sources. Blood also
presents an algorithm which allows the orientation, but not the
position, of the receiver relative to the transmitter to be
calculated non-iteratively.
Systems similar to Blood's are useful for tracking a probe, such as
a catheter or an endoscope, as that probe is moved through the body
of a medical patient. It is particularly important in this
application that the receiver be inside the probe and that the
transmitter be external to the patient, because transmitting
antennas of sufficient power would not fit inside the confined
volume of the probe. A representative prior art system of this type
is described in PCT Publication WO 96/05768, entitled "Medical
Diagnosis, Treatment and Imaging Systems", which is incorporated by
reference for all purposes as if fully set forth herein. Medical
applications of such systems include cismyocardial
revascularization, balloon catheterization, stent emplacement,
electrical mapping of the heart and the insertion of nerve
stimulation electrodes into the brain.
Perhaps the most important application of this tracking is to
intrabody navigation, as described by Acker in U.S. Pat. No.
5,729,129, with reference to PCT Publication No. WO 95/09562. A
three-dimensional image, such as a CT or MRI image, of the patient
is acquired. This image includes fiducial markers at predetermined
fiducial points on the surface of the patient. Auxiliary receivers
similar to the receiver of the probe are placed at the fiducial
points. The signals received by the auxiliary receivers are used to
register the image with respect to the transmitter frame of
reference, so that an icon that represents the probe can be
displayed, superposed on a slice of the image, with the correct
position and orientation with respect to the image. In this way, a
physician can see the position and orientation of the probe with
respect to the patient's organs.
WO 96/05768 illustrates another constraint imposed on such systems
by the small interior dimensions of the probe. In most prior art
systems, for example, the system of Egli et al., the receiver
sensors are three concentric orthogonal coils wound on a ferrite
core. The coils are "concentric" in the sense that their centers
coincide. Such a receiver of sufficient sensitivity would not fit
inside a medical probe. Therefore, the sensor coils of WO 96/05768
are collinear: the three orthogonal coils are positioned one behind
the other, with their centers on the axis of the probe, as
illustrated in FIG. 3 of WO 96/05768. This reduces the accuracy of
the position and orientation measurements, because instead of
sensing three independent magnetic field components at the same
point in space, this receiver senses three independent magnetic
field components at three different, albeit closely spaced, points
in space.
A further, consequent concession of the system of WO 96/05768 to
the small interior dimensions of a catheter is the use of coils
wound on air cores, rather than the conventional ferrite cores. The
high mutual coupling of collinear coils wound on ferrite cores and
measuring three independent field components at three different
points in space would distort those measurements sufficiently to
make those measurements fatally nonrepresentative of measurements
at a single point.
Another drawback of the system of WO 96/05768 relates to the
geometry of the transmitter antennas. These are three
nonoverlapping flat coplanar coils, preferably arranged in a
triangle. Because the strength of the field transmitted by one of
these coils falls as the reciprocal cube of the distance from the
coil, the receiver usually senses fields of very disparate
strength, which further degrades the accuracy of the position and
orientation measurements. Acker addresses this problem by
automatically boosting the power supplied to transmitting coils far
from the receiver. In U.S. Pat. No. 5,752,513, Acker et al. address
this problem by overlapping the coplanar transmitting coils.
Acker et al. transmit time-multiplexed DC signals. This time
multiplexing slows down the measurement. Frequency multiplexing, as
taught in WO 96/05768, overcomes this problem, but introduces a new
problem insofar as the transmitting coils are coupled by mutual
inductance at non-zero transmission frequency, so that the
transmitted field geometry is not the simple geometry associated
with a single coil, but the more complex geometry associated with
several coupled coils. This complicates and slows down the
calculation of the position and orientation of the receiver
relative to the transmitter coils. PCT Publication WO 97/36143,
entitled "Mutual Induction Correction", addresses this problem by
generating, at each transmitter coil, counter-fields that cancel
the fields generated by the other transmitter coils.
A further source of slowness in calculating the position and
orientation of the receiver is the iterative nature of the
calculation required for a spatially extended transmitter. As noted
above, Blood calculates the position of the receiver iteratively.
Even in the DC case, Acker et al. calculate both the position and
the orientation of the receiver iteratively.
There is thus a widely recognized need for, and it would be highly
advantageous to have, a faster and more accurate method for
tracking a medical probe inside the body of a patient.
SUMMARY OF THE INVENTION
According to the present invention there is provided a system for
tracking a position and an orientation of a probe, including a
plurality of first sensors, each of the first sensors for detecting
a different component of a vector force field, each of the first
sensors including two sensor elements disposed symmetrically about
a common reference point in the probe, the first sensors being
mounted inside the probe.
According to the present invention there is provided a method for
determining a position and an orientation of an object with respect
to a reference frame, including the steps of: (a) providing the
object with three independent sensors of electromagnetic radiation;
(b) providing three independent transmitting antennas of the
electromagnetic radiation, each of the transmitting antennas having
a fixed position in the reference frame, at least one of the
transmitting antennas being spatially extended; (c) transmitting
the electromagnetic radiation, using the transmitting antennas, a
first of the transmitting antennas transmitting the electromagnetic
radiation of a first spectrum, a second of the transmitting
antennas transmitting the electromagnetic radiation of a second
spectrum independent of the first spectrum, and a third of the
transmitting antennas transmitting the electromagnetic radiation of
a third spectrum independent of the first spectrum; (d) receiving
signals corresponding to the electromagnetic radiation, at all
three of the sensors, at a plurality of times, in synchrony with
the transmitting of the electromagnetic radiation; and (e)
inferring the position and the orientation of the object
noniteratively from the signals.
According to the present invention there is provided a system for
determining a position and an orientation of an object, including:
(a) a plurality of at least partly overlapping transmitter
antennas; (b) a mechanism for exciting the transmitter antennas to
transmit electromagnetic radiation simultaneously, the
electromagnetic radiation transmitted by each of the transmitter
antennas having a different spectrum; (c) at least one
electromagnetic field sensor, associated with the object, operative
to produce signals corresponding to the electromagnetic radiation;
and (d) a mechanism for inferring the position and the orientation
of the object from the signals.
According to the present invention there is provided a system for
determining a position and an orientation of an object, including:
(a) a plurality of at least partly overlapping transmitter
antennas; (b) a mechanism for exciting each of the transmitter
antennas to transmit electromagnetic radiation of a certain single
independent frequency and phase, the mechanism including, for each
of the transmitter antennas, a mechanism for decoupling the each
transmitter antenna from the electromagnetic radiation transmitted
by every other transmitter antenna; (c) at least one
electromagnetic field sensor, associated with the object, operative
to produce signals corresponding to the electromagnetic radiation;
and (d) a mechanism for inferring the position and the orientation
of the object from the signals.
According to the present invention there is provided a catheter,
including: (a) a housing having a transverse inner dimension of at
most about two millimeters; and (b) at least one coil, wound about
a solid core, mounted inside the housing.
According to the present invention there is provided a system for
navigating a probe inside a body, including: (a) a receiver of
electromagnetic radiation, inside the probe; (b) a device for
acquiring an image of the body; and (c) a transmitter, of the
electromagnetic radiation, including at least one antenna rigidly
attached to the device so as to define a frame of reference that is
fixed with respect to the device.
According to the present invention there is provided a system for
navigating a probe inside a body, including: (a) a first receiver
of electromagnetic radiation, inside the probe; (b) a device for
acquiring an image of the body; and (c) a second receiver, of the
electromagnetic radiation, rigidly attached to the device so as to
define a frame of reference that is fixed with respect to the
device.
According to the present invention there is provided a method of
navigating a probe inside a body, including the steps of: (a)
providing a device for acquiring an image of the body; (b)
simultaneously: (i) acquiring the image of the body, and (ii)
determining a position and orientation of the probe with respect to
the image; and (c) displaying the image of the body with a
representation of the probe superposed thereon according to the
position and the orientation.
According to the present invention there is provided a device for
sensing an electromagnetic field at a point, including at least
four sensing elements, at least two of the sensing elements being
disposed eccentrically with respect to the point.
According to the present invention there is provided a method for
determining a position and an orientation of an object with respect
to a reference frame, including the steps of: (a) providing the
object with three independent sensors of electromagnetic radiation;
(b) providing three independent transmitting antennas of the
electromagnetic radiation, each of the transmitting antennas having
a fixed position in the reference frame, at least one of the
transmitting antennas being spatially extended; (c) transmitting
the electromagnetic radiation, using the transmitting antennas, a
first of the transmitting antennas transmitting the electromagnetic
radiation of a first spectrum, a second of the transmitting
antennas transmitting the electromagnetic radiation of a second
spectrum independent of the first spectrum, and a third of the
transmitting antennas transmitting the electromagnetic radiation of
a third spectrum independent of the first spectrum; (d) receiving
signals corresponding to the electromagnetic radiation, at all
three of the sensors, at a plurality of times, in synchrony with
the transmitting of the electromagnetic radiation; (e) setting up
an overdetermined set of linear equations relating the signals to a
set of amplitudes, there being, for each of the sensors: for each
transmitting antenna: one of the amplitudes; and (f) solving the
set of linear equations for the amplitudes.
According to the present invention there is provided a method of
navigating a probe inside a body, including the steps of: (a)
providing a device for acquiring an image of the body; (b)
simultaneously: (i) acquiring the image of the body, and (ii)
determining a position and an orientation of the body with respect
to the image; (c) determining a position and an orientation of the
probe with respect to the body; and (d) displaying the image of the
body with a representation of the probe superposed thereon
according to both of the positions and both of the
orientations.
According to the present invention there is provided a device for
sensing an electromagnetic field at a point, including: (a) two
sensing elements, each of the sensing elements including a first
lead and a second lead, the first leads being electrically
connected to each other and to ground; and (b) a differential
amplifier, each of the second leads being electrically connected to
a different input of the differential amplifier.
According to the present invention there is provided a catheter
including: (a) an outer sleeve having an end; (b) an inner sleeve
having an end and slidably mounted within the outer sleeve; (c) a
first flexible member connecting the end of the outer sleeve to the
end of the inner sleeve; and (d) a first coil mounted on the first
flexible member.
According to the present invention there is provided a system for
determining a position and an orientation of an object,
including:(a) at least one transmitter antenna for transmitting an
electromagnetic field; (b) a first electromagnetic field sensor,
associated with the object and including two sensing elements
responsive to a first component of the transmitted electromagnetic
field, each of the sensing elements including a first lead and a
second lead, the first leads being electrically connected to each
other and to ground; and (c) a first differential amplifier, each
of the second leads being electrically connected to a different
input of the first differential amplifier.
According to the present invention there is provided an imaging
device, including: (a) an electrically conducting surface; (b) a
magnetically permeable compensator; and (c) a mechanism for
securing the compensator relative to the surface so as to
substantially suppress a distortion of an external electromagnetic
field caused by the surface.
According to the present invention there is provided a device for
sensing an electromagnetic field, including: (a) a housing,
including a first pair of diametrically opposed apertures, (b) a
first core mounted in the first pair of apertures; and (c) a first
coil of electrically conductive wire wound about the core.
According to the present invention there is provided a probe for
interacting with a body cavity, including: (a) a substantially
cylindrical catheter; (b) a satellite; and (c) a mechanism for
reversibly securing the satellite at a fixed position and
orientation relative to the catheter after the catheter and the
satellite have been inserted into the body cavity.
Each receiver sensor of the present invention includes two sensor
elements placed symmetrically with respect to a reference point
inside the probe. All the sensor element pairs share the same
reference point, so that the measured magnetic field components are
representative of the field component values at the single
reference point, instead of at three different points, as in the
prior art system, despite the confined transverse interior
dimensions of the probe. Because of the symmetric disposition of
the sensor elements with respect to the reference point, the
measured magnetic field components are representative of the field
components at the reference point, despite the individual sensing
elements not being centered on the reference point. This property
of not being centered on the reference point is termed herein an
eccentric disposition with respect to the reference point.
In one preferred embodiment of the receiver of the present
invention, the sensor elements are helical coils. Within each
sensor, the coils are mutually parallel and connected in series. As
in the case of the prior art receivers, the coils are arranged with
their centers on the axis of the probe. To ensure that coils of
different sensors are mutually perpendicular, the probe housing
includes mutually perpendicular pairs of diametrically opposed
apertures formed therein, the coils whose axes are perpendicular to
the axis of the probe are wound about cores whose ends extend past
the ends of the respective coils, and the ends of the cores are
mounted in their respective apertures.
In another preferred embodiment of the receiver of the present
invention, with three sensors, the sensor elements are flat
rectangular coils bent to conform to the shape of the cylindrical
interior surface of the probe. The sensor elements of the three
sensors are interleaved around the cylindrical surface. The
advantage of this preferred embodiment over the first preferred
embodiment is that this preferred embodiment leaves room within the
probe for the insertion of other medical apparati.
As noted above, within any one sensor, the coils are connected in
series. This connection is grounded. The other end of each coil is
connected, by one wire of a twisted pair of wires, to a different
input of a differential amplifier.
In a preferred embodiment of a cardiac catheter that incorporates a
receiver of the present invention, the catheter includes an inner
sleeve mounted slidably within an outer sleeve. One of the sensors
includes two coils mounted within the inner sleeve, towards the
distal end of the catheter. The distal end of the inner sleeve is
connected to the distal end of the outer sleeve by flexible strips.
Each of the other sensors includes two coils mounted on opposed
lateral edges of a pair of flexible strips that flank the inner
sleeve, with the inner sleeve running between the two members of
the pair. When the inner sleeve is in the extended position thereof
relative to the outer sleeve, the flexible strips lie flat against
the inner sleeve, and the catheter can be maneuvered towards a
patient's heart via the patient's blood vessels. When the end of
the catheter has been introduced to the targeted chamber of the
heart, the inner sleeve is withdrawn to the retracted position
thereof relative to the outer sleeve, and the pairs of flexible
strips form circles that are concentric with the reference point.
Also mounted on the outward-facing surfaces of the flexible strips
and, optionally, on the distal end of the inner sleeve, are
electrodes for electrophysiologic mapping of the heart.
Alternatively, the electrode on the distal end of the inner sleeve
may be used for ablation of cardiac tissue, for example in the
treatment of ventricular tachycardia.
An alternative preferred embodiment of the cardiac catheter of the
present invention has an inflatable balloon connecting the distal
ends of the inner and outer sleeves. The coils of the external
sensors are mounted on the external surface of the balloon. When
the inner sleeve is in the extended position thereof relative to
the outer sleeve, the balloon lies flat against the inner sleeve,
and the catheter can be maneuvered towards the patient's heart via
the patient's blood vessels. When the end of the catheter has been
introduced to the targeted chamber of the heart, the inner sleeve
is withdrawn to the retracted position thereof relative to the
outer sleeve, and the balloon is inflated to a sphere that is
concentric with the reference point.
Although the primary application of the receiver of the present
invention is to tracking a probe by receiving externally generated
electromagnetic radiation, the scope of the present invention
includes receivers for similar tracking based on the reception of
any externally generated vector force field, for example, a time
varying isotropic elastic field.
The algorithm of the present invention for inferring the position
and orientation of the receiver with respect to the transmitter is
similar to the algorithm described in co-pending Israel Patent
Application 122578. The signals received by the receiver are
transformed to a 3.times.3 matrix M. The columns of M correspond to
linear combinations of the amplitudes of the transmitted fields.
The rows of M correspond to the receiver sensors. A rotationally
invariant 3.times.3 position matrix W and a 3.times.3 rotation
matrix Tare inferred noniteratively from the matrix M. The Euler
angles that represent the orientation of the receiver relative to
the transmitter antennas are calculated noniteratively from the
elements of T, and the Cartesian coordinates of the receiver
relative to the transmitter antennas are calculated from the
elements of W A preliminary calibration of the system, either by
explicitly measuring the signals received by the receiver sensors
at a succession of positions and orientations of the receiver, or
by theoretically predicting these signals at the successive
positions and orientations of the receiver, is used to determine
polynomial coefficients that are used in the noniterative
calculation of the Euler angles and the Cartesian coordinates. In
essence, the extra time associated with an iterative calculation is
exchanged for the extra time associated with an initial
calibration. One simplification of the algorithm of the present
invention, as compared to the algorithm of IL 122578, derives from
the fact that the system of the present invention is a closed loop
system.
The preferred arrangement of the transmitter antennas of the
present invention is as a set of flat, substantially coplanar coils
that at least partially overlap. Unlike the preferred arrangement
of Acker et al., it is not necessary that every coil overlap every
other coil, as long as each coil overlaps at least one other coil.
The most preferred arrangement of the transmitter antennas of the
present invention consists of three antennas. Two of the antennas
are adjacent and define a perimeter. The third antenna partly
follows the perimeter and partly overlaps the first two antennas.
The elements of the first column of M are sums of field amplitudes
imputed to the first two antennas. The elements of the second
column of M are differences of field amplitudes imputed to the
first two antennas. The elements of the third column of M are
linear combinations of the field amplitudes imputed to all three
antennas that correspond to differences between the field
amplitudes imputed to the third antenna and the field amplitudes
that would be imputed to a fourth antenna that overlaps the portion
of the first two antennas not overlapped by the third antenna.
The signals transmitted by the various antennas of the present
invention have different, independent spectra. The term "spectrum",
as used herein, encompasses both the amplitude and the phase of the
transmitted signal, as a function of frequency. So, for example, if
one antenna transmits a signal proportional to cos .omega.t and
another antenna transmits a signal proportional to sin .omega.t,
the two signals are said to have independent frequency spectra
because their phases differ, even though their amplitude spectra
both are proportional to .delta.(.omega.). The term "independent
spectra", as used herein, means that one spectrum is not
proportional to another spectrum. So, for example, if one antenna
transmits a signal equal to cos.omega.t and another antenna
transmits a signal equal to 2 cos .omega.t, the spectra of the two
signals are not independent. Although the scope of the present
invention includes independent transmitted signals that differ only
in phase, and not in frequency, the examples given below are
restricted to independent transmitted signals that differ in their
frequency content.
The method employed by the present invention to decouple the
transmitting antennas, thereby allowing each antennas to transmit
at only a single frequency different from the frequencies at which
the other antennas transmit, or, alternatively, allowing two
antennas to transmit at a single frequency but with a predetermined
phase relationship between the two signals, is to drive the
antennas with circuitry that makes each antenna appear to the
fields transmitted by the other antennas as an open circuit. To
accomplish this, the driving circuitry of the present invention
includes active circuit elements such as differential amplifiers,
unlike the driving circuitry of the prior art, which includes only
passive elements such as capacitors and resistors. By "driving
circuitry" is meant the circuitry that imposes a current of a
desired transmission spectrum on an antenna, and not, for example,
circuitry such as that described in WO 97/36143 whose function is
to detect transmissions by other antennas with other spectra and
generate compensatory currents.
With respect to intrabody navigation, the scope of the present
invention includes the simultaneous acquisition and display of an
image of the patient and superposition on that display of a
representation of a probe inside the patient, with the
representation positioned and oriented with respect to the image in
the same way as the probe is positioned and oriented with respect
to the patient. This is accomplished by positioning and orienting
the imaging device with respect to the frame of reference of the
transmitter, in one of two ways. Either the transmitter antennas
are attached rigidly to the imaging device, or a second receiver is
attached rigidly to the imaging device and the position and
orientation of the imaging device with respect to the transmitter
are determined in the same way as the position and orientation of
the probe with respect to the transmitter are determined. This
eliminates the need for fiducial points and fiducial markers. The
scope of the present invention includes both 2D and 3D images, and
includes imaging modalities such as CT, MRI, ultrasound and
fluoroscopy. Medical applications to which the present invention is
particularly suited include transesophageal echocardiography,
intravascular ultrasound and intracardial ultrasound. In the
context of intrabody navigation, the term "image" as used herein
refers to an image of the interior of the patient's body, and not
to an image of the patient's exterior.
Under certain circumstances, the present invention facilitates
intrabody navigation even if the image is acquired before the probe
is navigated through the patient's body with reference to the
image. A third receiver is attached rigidly to the limb of the
patient to which the medical procedure is to be applied. During
image acquisition, the position and orientation of the third
receiver with respect to the imaging device is determined as
described above. This determines the position and orientation of
the limb with respect to the image. Subsequently, while the probe
is being moved through the limb, the position and orientation of
the probe with respect to the limb is determined using the second
method described above to position and orient the probe with
respect to the imaging device during simultaneous imaging and
navigation. Given the position and orientation of the probe with
respect to the limb and the orientation and position of the limb
with respect to the image, it is trivial to infer the position and
orientation of the probe with respect to the image.
Many imaging devices used in conjunction with the present invention
include electrically conducting surfaces. One important example of
such an imaging device is a fluoroscope, whose image intensifier
has an electrically conducting front face. According to the present
invention, the imaging device is provided with a magnetically
permeable compensator to suppress distortion of the electromagnetic
field near the electrically conducting surface as a consequence of
eddy currents induced in the electrically conducting surface by the
electromagnetic waves transmitted by the transmitting antennas of
the present invention.
The scope of the present invention includes a scheme for
retrofitting an apparatus such as the receiver of the present
invention to a catheter to produce an upgraded probe for
investigating or treating a body cavity of a patient. A tether
provides a loose mechanical connection between the apparatus and
the catheter while the apparatus and the catheter are inserted into
the patient. When the apparatus and the catheter reach targeted
body cavity, the tether is withdrawn to pull the apparatus into a
pocket on the catheter. The pocket holds the apparatus in a fixed
position and orientation relative to the catheter.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with
reference to the accompanying drawings, wherein:
FIG. 1 is a schematic diagram of a system of the present
invention;
FIG. 2A is a partly cut away perspective view of a probe and a
receiver;
FIG. 2B is a circuit diagram of the receiver of FIG. 2A;
FIG. 2C illustrates features of the receiver of FIG. 2A that
suppress unwanted electromagnetic coupling;
FIG. 3 is an axial sectional view of a probe and a receiver;
FIG. 4A shows two coils of opposite helicities;
FIG. 4B shows two coils of identical helicities;
FIG. 5 shows a second preferred embodiment of a receiver;
FIG. 6 is a plan view of three loop antennas and two phantom loop
antennas;
FIGS. 7A, 7B and 7C show alternative configurations of paired
adjacent loop antennas;
FIG. 8 is a schematic block diagram of driving circuitry
FIG. 9 shows a C-mount fluoroscope modified for real-time intrabody
navigation
FIG. 10 shows a coil of the receiver of FIG. 5;
FIG. 11 shows a CT scanner modified for imaging in support of
subsequent intracranial navigation;
FIG. 12A is a partly cut-away perspective view of a cardiac
catheter of the present invention in the retracted position
thereof;
FIG. 12B is a perspective view of the catheter of FIG. 12A in the
extended position thereof;
FIG. 12C is an end-on view of the catheter of FIG. 12a in the
retracted position thereof;
FIG. 13A is a partly cut-away side view of a second embodiment of
the cardiac catheter of the present invention in the retracted and
inflated position thereof;
FIG. 13B is an end-on view of the catheter of FIG. 13A in the
retracted and inflated position thereof;
FIG. 14 is a partial perspective view of the C-mount fluoroscope of
FIG. 9, including a magnetically permeable compensator;
FIG. 15 is a partial exploded perspective view of a preferred
embodiment of the probe and receiver of FIG. 2A;
FIG. 16 illustrates a scheme for retrofitting an apparatus such as
the receiver of FIG. 2A to a catheter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is of a system and method for tracking the
position and orientation of an object relative to a fixed frame of
reference. Specifically, the present invention can be used to track
the motion of a medical probe such as a catheter or an endoscope
within the body of a patient.
The principles and operation of remote tracking according to the
present invention may be better understood with reference to the
drawings and the accompanying description.
Referring now to the drawings, FIG. 1 illustrates, in general
terms, a system of the present invention. Within a probe 10 is
rigidly mounted a receiver 14. Receiver 14 includes three field
component sensors 16, 18, and 20, each for sensing a different
component of an electromagnetic field. Sensor 16 includes two
sensor elements 16a and 16b. Sensor 18 includes two sensor elements
18a and 18b. Sensor 20 includes two sensor elements 20a and 20b.
Typically, the sensor elements are coils, and the sensed components
are independent magnetic field components. Sensor elements 16a and
16b are on opposite sides of, and equidistant from, a common
reference point 22. Similarly, sensor elements 18a and 18b are on
opposite sides of, and equidistant from, point 22, and sensor
elements 20a and 20b also are on opposite sides of, and equidistant
from, point 22. In the illustrated example, sensors 16, 18 and 20
are disposed collinearly along a longitudinal axis 12 of probe 10,
but other configurations are possible, as discussed below.
The system of FIG. 1 also includes a transmitter 24 of
electromagnetic radiation. Transmitter 24 includes three
substantially coplanar rectangular loop antennas 26, 28 and 30
connected to driving circuitry 32. Loop antennas 26 and 28 are
adjacent and are partly overlapped by loop antenna 30. Driving
circuitry 32 includes appropriate signal generators and amplifiers
for driving each of loop antennas 26, 28 and 30 at a different
frequency. The electromagnetic waves generated by transmitter 24
are received by receiver 14. The signals from receiver 14 that
correspond to these electromagnetic waves are sent to reception
circuitry 34 that includes appropriate amplifiers and A/D
converters. Reception circuitry 34 and driving circuitry 32 are
controlled by a controller/processor 36 that typically is an
appropriately programmed personal computer. Controller/processor 36
directs the generation of transmitted signals by driving circuitry
32 and the reception of received signals by reception circuitry 34.
Controller/processor 36 also implements the algorithm described
below to infer the position and orientation of probe 10. Note that
the system of FIG. 1 is a closed-loop system: the reception of
signals from receiver 14 is synchronized with the transmission of
electromagnetic waves by transmitter 24.
FIG. 2 shows a particular, slightly modified embodiment of receiver
14. FIG. 2A is a perspective, partly cut away view of probe 10 with
receiver 14 mounted in the housing 11 thereof. FIG. 2B is a circuit
diagram of receiver 14. In this embodiment, sensor elements 16a,
16b, 18a and 18b are coils of conducting wire wound on ferrite
cores 70. Coils 16a and 16b are mutually parallel. Coils 18a and
18b are mutually parallel and are perpendicular to coils 16a and
16b. Coils 16a, 16b, 18a and 18b all are perpendicular to axis 12.
Instead of sensor 20 with two sensor elements 20a and 20b, the
embodiment of FIG. 2 has a single coil 20' of conducting wire wound
on a ferrite core 70. Coil 20' is parallel to axis 12 and therefore
is perpendicular to coils 16a, 16b, 18a and 18b. Coil 20' is
centered on reference point 22. Sensors 16, 18 and 20' are
connected to reception circuitry 34 by twisted wire pairs 38. As
shown in the circuit diagram of FIG. 2B, coils 16a and 16b are
connected in series, and coils 18a and 18b are connected in
series.
Because sensors 16, 18 and 20' of FIG. 2 all measure field
components at the same reference point 22, coils 16a, 16b, 18a, 18b
and 20' can be wound on ferrite cores 70 instead of the air cores
of WO 96/05768 without causing undue distortion of the received
signals, despite the small transverse interior diameter 72,
typically less than two millimeters, of probe 10 when probe 10 is a
catheter.
Wire pairs 38 are twisted in order to suppress electromagnetic
coupling between wire pairs 38 and the environment, and in
particular to suppress electromagnetic coupling between wire pairs
38 and transmitter 24. FIG. 2C is a circuit diagram that shows
further features of the present invention that suppress this
electromagnetic coupling. FIG. 2C is drawn with particular
reference to sensor 16, but the same features apply, mutatis
mutandis, to sensor 18.
Coils 16a and 16b are connected in series by inner leads 116a and
116b thereof. Outer lead 216a of coil 16a is connected, by wire 38a
of twisted wire pair 38, to a positive input 126a of a differential
amplifier 128 of reception circuitry 34. Outer lead 216b of coil
16b is connected, by wire 38b of twisted wire pair 38, to a
negative input 126b of differential amplifier 128. Inner leads 116a
and 116b also are connected to ground 124 by a wire 122. For
illustrational clarity, wire 38a is drawn as a solid line, wire 38b
is drawn as a dotted line and wire 122 is drawn as a dashed
line.
FIG. 15 is a partial exploded perspective view of a preferred
embodiment of probe 10 and receiver 14. Housing 11 is substantially
cylindrical, with two recesses 511 and 513 incised therein. The
boundary of each recess 511 or 513 includes a pair of diametrically
opposed apertures: apertures 510 and 512 in the boundary of recess
511 and apertures 514 and 516 in the boundary of recess 513. Arrows
530 and 532 show two of the three components of a cylindrical
coordinate system for describing position within and along housing
11. Arrow 530 points in the longitudinal direction. Arrow 532
points in the azimuthal direction. Aperture pair 510, 512 is
displaced both longitudinally and azimuthally from aperture pair
514, 516.
Coil 16a is a coil of electrically conducting wire that is wound
about a core 70a. Core 70a is mounted in apertures 514 and 516: end
518 of core 70a, that extends beyond coil 16a, is mounted in
aperture 514 and is secured rigidly in place by a suitable glue,
and end 520 of core 70a, that extends beyond coil 16a in the
opposite direction, is mounted in aperture 516 and is secured
rigidly in place by a suitable glue. Similarly, coil 18a is a coil
of electrically conducting wire that is wound about a core 70b.
Core 70b is mounted in apertures 510 and 512: end 522 of core 70b,
that extends beyond coil 18a, is mounted in aperture 510 and is
secured rigidly in place by a suitable glue, and end 524 of core
70b, that extends beyond coil 18a in the opposite direction, is
mounted in aperture 512 and is secured rigidly in place by a
suitable glue.
FIG. 15 also shows the preferred azimuthal separation of aperture
pair 514, 516 from aperture pair 510, 512. Aperture pair 514, 516
is perpendicular to aperture pair 510, 512, in the sense that
aperture pair 514, 516 is displaced 90.degree., in the direction of
arrow 532, from aperture pair 510, 512. This makes core 70a
perpendicular to core 70b, and hence makes coils 16a and 18a
mutually perpendicular.
In the case of probe 10 being a catheter for invasively probing or
treating a body cavity such as a chamber of the heart, it is
preferable that housing 11 be made of a nonmagnetic metal such as
nitinol, titanium, iconel, phynox or stainless steel. Housing 11
thus is sufficiently flexible to bend under the lateral forces of
the walls of blood vessels through which probe 10 is inserted
towards the body cavity, and sufficiently resilient to return to
its unstressed shape, with coils 16a and 18a mutually
perpendicular, when the portion of probe 10 that includes receiver
14 reaches the interior of the body cavity. Surprisingly, it has
been found that the use of a conductive metal as the material of
housing 11 does not distort the electromagnetic field sensed by
receiver 14 despite the current eddies induced in housing 11 by the
electromagnetic waves generated by transmitter 24. Apertures 510,
512, 514 and 516 are most conveniently formed by laser cutting. The
accuracy of the mutual perpendicularity of coils 16a and 18a
obtained in this manner has been found to be superior to the
accuracy obtained by forming housing 11 as a solid cylindrical
block and drilling mutually perpendicular recesses in the block to
receive coils 16a and 18a.
Coils 16b and 18b are mounted similarly in similar pairs of
diametrically opposed, azimuthally and longitudinally displaced
apertures. This ensures that coils 16a and 16b are mutually
parallel, that coils 18a and 18b are mutually parallel, and that
coils 16b and 18b are mutually perpendicular.
In an alternative structure (not shown) of housing 11, housing 11
is formed as an open, spring-like frame that includes apertures
510, 512, 514 and 516 in the form of small rings that are sized to
accept the ends 518, 520, 522 and 524 of cores 70a and 70b. The
spring-like nature of this embodiment of housing 11 allows coils
16a and 18a to be mounted therein simply by forcing ends 518, 520,
522 and 524 into their respective apertures, and also allows
housing 11 to flex during insertion towards a body cavity of a
patient and to return to its unstressed shape upon arrival inside
the body cavity.
FIG. 3 is an axial sectional view of receiver 14 mounted in a
variant of probe 10 that has two sections 10a and 10b connected by
a flexible connector 40. As in FIG. 2, sensors 16 and 18 include
sensor elements 16a, 16b, 18a and 18b that are coils of conducting
wire wound on air cores and that are perpendicular to axis 12.
Sensor elements 16a and 16b are mutually parallel, sensor elements
18a and 18b are mutually parallel, and sensor elements 16a and 16b
are perpendicular to sensor elements 18a and 18b. Sensor 20
includes two sensor elements: coils 20a and 20b of conducting wire
wound on air cores. Coils 20a and 20b are equidistant from
reference point 22 and are parallel to axis 12. Like coils 16a and
16b and like coils 18a and 18b, coils 20a and 20b are connected in
series. Flexible connector 40 allows this variant of probe 10 to
bend as this variant of probe 10 is moved within a medical patient.
Sensor element pairs 16, 18 and 20 are disposed symmetrically with
respect to reference point 22 in the sense that when probe 10 of
FIG. 3 is straight, as drawn, sensor elements 16a and 16b are on
opposite sides of, and equidistant from, reference point 22; and
likewise sensor elements 18a and 18b are on opposite sides of, and
are equidistant from, reference point 22; and sensor elements 20a
and 20b are on opposite sides of, and are equidistant from,
reference point 22. Note that when probe 10 of FIG. 3 is straight,
sensor elements 16a, 16b, 18a, 18b, 20a and 20b all are collinear,
along axis 12 that intersects point 22, and so are disposed
symmetrically with respect to point 22.
For coil pairs such as pairs 16a and 16b to produce signals
representative of a magnetic field component at point 22 when the
coil pairs are connected as shown in FIG. 2A, the two coils must
have opposite helicity, as illustrated in FIG. 4A, so that, in a
spatially uniform time varying magnetic field, the signals induced
in the two coil pairs 16a and 16b reinforce each other instead of
canceling each other. Coil pairs 16a and 16b that have identical
helicities, as illustrated in FIG. 4B, may be used to measure a
magnetic field component gradient at point 22. Alternatively, coil
pairs of identical helicities may be used to measure magnetic field
components if the top of one coil is connected to the bottom of the
other coil.
FIG. 5 illustrates a second class of preferred embodiments of
receiver 14. In FIG. 5, a conceptual cylindrical surface is denoted
by dashed lines 42 and dashed circles 44. The embodiment of
receiver 14 illustrated in FIG. 5 includes three sensors 16, 18 and
20, each with two sensor elements 16c and 16d, 18c and 18d, and 20c
and 20d, respectively. Each sensor element is a flat rectangular
coil, of many turns of conducting wire, that is bent into an
arcuate shape to conform to the shape of the cylindrical surface.
Sensor elements 16c, 18c and 20c are interleaved around circle 44a.
Sensor elements 16d, 18d and 20d are interleaved around circle 44b.
Sensor elements 16c and 16d are disposed symmetrically with respect
to reference point 22, meaning that sensor elements 16c and 16d are
on opposite sides of reference point 22, are equidistant from
reference point 22, and are oriented so that an appropriate
180.degree. rotation about point 22 maps sensor 16c into sensor
16d. Similarly, sensor elements 18c and 18d are disposed
symmetrically with respect to reference point 22, and sensor
elements 20c and 20d are disposed symmetrically with respect to
reference point 22. Sensor elements 16c and 16d are connected in
series, in a manner similar to sensor elements 16a and 16b, to
respond to one component of the magnetic field. Sensor elements 18c
and 18d are connected similarly in series to respond to a second
component of the magnetic field that is independent of the first
component, and sensor elements 20c and 20d are connected similarly
in series to respond to a third component of the magnetic field
that is independent of the first two components. Most preferably,
sensor elements 16c, 16d, 18c, 18d, 20c and 20d are sized and
separated so that these three magnetic field components are
orthogonal. In practice, the cylindrical surface whereabout sensor
elements 16c, 16d, 18c, 18d, 20c and 20d are disposed could be the
inner surface of probe 10 or the outer surface of a cylindrical
sleeve adapted to fit inside probe 10. In the case of this
embodiment of receiver 14 formed on the outer surface of a
cylindrical sleeve, sensor elements 16c, 16d, 18c, 18d, 20c and 20d
may be fabricated by any one of several standard methods, including
photolithography and laser trimming. FIG. 10 illustrates the
preferred geometry of sensor elements 16c, 16d, 18c, 18d, 20c and
20d: a flat rectangular spiral 17 of an electrical conductor 19.
Only four turns are shown in spiral 17, for illustrational
simplicity. Preferably, however, there are several hundred turns in
spiral 17. For example, a spiral 17, intended for a cylindrical
surface of a diameter of 1.6 millimeters, in which conductor 19 has
a width of 0.25 microns, and in which the windings are separated by
gaps of 0.25 microns, has 167 turns.
FIGS. 12A, 12B and 12C illustrate the distal end of a cardiac
catheter 300 of the present invention. FIG. 12A is a partly
cut-away perspective view of catheter 300 in the retracted position
thereof. FIG. 12B is a perspective view of catheter 300 in the
extended position thereof. FIG. 12C is an end-on view of catheter
300 in the retracted position thereof. Catheter 300 includes a
flexible cylindrical inner sleeve 302 slidably mounted in a
flexible cylindrical outer sleeve 304. Connecting distal end 306 of
inner sleeve 302 to distal end 308 of outer sleeve 304 are four
flexible rectangular strips 310. When inner sleeve 302 is in the
extended position thereof relative to outer sleeve 304, strips 310
are flush against inner sleeve 302, as shown in FIG. 12B. When
inner sleeve 302 is in the retracted position thereof relative to
outer sleeve 304, strips 310 bow outward in circular arcs, as shown
in FIG. 12A.
Catheter 300 includes a set of three orthogonal electromagnetic
field component sensors 316, 318 and 320, in the manner of receiver
14 of FIG. 1. First sensor 316 includes coils 316a and 316b mounted
on opposite lateral edges 312a and 314a of strip 310a and on
opposite lateral edges 312c and 314c of strip 310c. Coil 316a is
mounted on lateral edges 312a and 312c. Coil 316b is mounted on
lateral edges 314a and 314b. Second sensor 318 includes coils 318a
and 318b mounted on opposite lateral edges 312b and 314b of strip
310b and on opposite lateral edges 312d and 314d of strip 310d.
Coil 318a is mounted on lateral edges 312b and 312d. Coil 318b is
mounted on lateral edges 314b and 314d. Third sensor 320 includes
coils 320a and 320b. Inner sleeve 302 is cut away in FIG. 12A to
show coils 320a and 320b. For illustrational clarity, the wires of
coils 316a and 318a are shown in FIGS. 12A and 12B as dashed lines,
and only two turns are shown for each coil, although in practice at
least nine turns of 45-micron-diameter copper wire are used. Note
that the wires of coil 316a run through inner sleeve 302, from
lateral edge 312a to lateral edge 312c, and do not terminate at the
intersection of lateral edges 312a and 312c with inner sleeve 302.
Similarly, the wires of coil 318a do not terminate at the
intersection of lateral edges 312b and 312d with inner sleeve 302,
but instead run from lateral edge 312b to lateral edge 312d. Also
for illustrational clarity, lateral edges 312 are shown much wider
than they really are in preferred embodiments of catheter 300.
Coils 320a and 320b are wound around a permeable core (not
shown).
In a typical embodiment of catheter 300, the length of inner sleeve
302 exceeds the length of outer sleeve 304 by 15.7 mm in the
extended position. Also in a typical embodiment of catheter 300,
each of coils 320a and 320b is about 1.1 mm long and about 1.1 mm
in diameter and includes about 400 turns of 10 micron diameter
copper wire.
Coils 320a and 320b are parallel and equidistant from a central
point 322. When catheter 300 is opened to the retracted position
thereof, as shown in FIGS. 12A and 12C, the circular arcs formed by
strips 310 are concentric with point 322. This makes coils 316a,
316b, 318a and 318b circular and concentric with point 322, with
coils 316a and 316b being mutually parallel, and with coils 318a
and 318b being mutually parallel, so that point 322 then becomes
the reference point for electromagnetic field measurements.
In the extended position thereof, catheter 300 is thin enough,
preferably less than about 2 mm in diameter, to be inserted via the
blood vessels of a patient into the patient's heart. Once the
distal end of catheter 300 is inside the desired chamber of the
patient's heart, inner sleeve 302 is withdrawn relative to outer
sleeve 304 to put catheter 300 in the retracted position thereof.
Sensors 316, 318 and 320 are used in conjunction with transmitter
24 in the manner described below to determine the location and
orientation of the distal end of catheter 300 within the patient's
heart.
Mounted on outward faces 324 of strips 310 are four electrodes 326.
Mounted on distal end 306 of inner sleeve 302 is an electrode 328.
Electrodes 326 and 328 may be used for electrophysiologic mapping
of the patient's heart. Alternatively, high RF power levels may be
applied to selected heart tissue via electrode 328 to ablate that
tissue in the treatment of conditions such as ventricular
tachycardia.
FIGS. 13A and 13B illustrate the distal end of an alternative
embodiment 400 of the cardiac catheter of the present invention.
FIG. 13A is a partly cut-away side view of catheter 400 in the
retracted position thereof. FIG. 13B is an end-on view of catheter
400 in the retracted position thereof. Like catheter 300, catheter
400 includes a flexible cylindrical inner sleeve 402 slidably
mounted in a flexible cylindrical outer sleeve 404. Connecting
distal end 406 of inner sleeve 402 to distal end 408 of outer
sleeve 404 is a single flexible member: an inflatable latex balloon
410. When inner sleeve 402 is in the extended position thereof
relative to outer sleeve 404, balloon 410 is flush against inner
sleeve 402. After the illustrated distal end of catheter 400 has
been introduced to the targeted chamber of a patient's heart, inner
sleeve 402 is withdrawn to the retracted position thereof, and
balloon 410 is inflated to assume a spherical shape.
Like catheter 300, catheter 400 includes a set of three orthogonal
electromagnetic field component sensors 416, 418 and 420, in the
manner of receiver 14 of FIG. 1. First sensor 416 includes parallel
coils 416a and 416b mounted as shown on outer surface 412 of
balloon 410. Second sensor 418 includes parallel coils 418a and
418b mounted orthogonally to coils 416a and 416b on outer surface
412, as shown. Third sensor 420 includes coils 420a and 420b.
Balloon 410 and inner sleeve 402 are cut away in FIG. 13A to show
coils 420a and 420b. Coils 420a and 420b are parallel and
equidistant from a central point 422. When catheter 400 is opened
to the retracted position thereof and balloon 410 is inflated to a
spherical shape, outer surface 412 is a sphere concentric with
point 422. This makes coils 416a, 416b, 418a and 418b circular and
concentric with point 422, so that point 422 then becomes the
reference point for electromagnetic field measurements.
Also as in the case of catheter 300, catheter 400 includes four
electrodes 426, similar to electrodes 326, mounted on outer surface
412, and an electrode 428, similar to electrode 328, mounted on
distal end 406 of inner sleeve 402.
FIG. 6 is a plan view of loop antennas 26, 28 and 30. Loop antenna
26 is a rectangle with legs 26a, 26b, 26c and 26d. Loop antenna 28
is a rectangle of the same shape and size as loop antenna 26, and
with legs 28a, 28b, 28c and 28d. Legs 26b and 28d are adjacent.
Loop antenna 30 also is rectangular, with legs 30a, 30b, 30c and
30d. Leg 30a overlies legs 26a and 28a; leg 30b overlies the upper
half of leg 28b; and leg 30d overlies the upper half of leg 26d, so
that loop antenna 30 overlaps half of loop antenna 26 and half of
loop antenna 28. Also shown in phantom in FIG. 6 is a fourth
rectangular loop antenna 46 and a fifth rectangular loop antenna 48
that are not part of transmitter 24 but are referred to in the
explanation below. Loop antenna 46 is of the same shape and size as
loop antenna 30, and overlaps the halves of loop antennas 26 and 28
that are not overlapped by loop antenna 30. Loop antenna 48 matches
the outer perimeter defined by loop antennas 26 and 28.
To understand the preferred mode of the operation of the system of
the present invention, it is helpful to consider first a less
preferred mode, based on time domain multiplexing, of operating a
similar system that includes all five loop antennas of FIG. 6. In
this less preferred mode, loop antenna 48 is energized using a
sinusoidal current of angular frequency .omega..sub.1. Then, loop
antennas 26 and 28 are energized by oppositely directed sinusoidal
currents of angular frequency .omega..sub.1. Finally, loop antennas
30 and 46 are energized by oppositely directed sinusoidal currents
of angular frequency .omega..sub.1. The idea of this energization
sequence is to produce, first, a field above the transmitter that
is spatially symmetric in both the horizontal and the vertical
direction as seen in FIG. 6, then a field above the transmitter
that is antisymmetric in the horizontal direction and symmetric in
the vertical direction, and finally a field that is symmetric in
the horizontal direction and antisymmetric in the vertical
direction. These three fields are linearly independent, and all
three fields have significant amplitude all the way across the
transmitter. The signals output by the three sensors of receiver 14
in response to the electromagnetic waves so generated are sampled
at times t.sub.m by reception circuitry 34. The sampled signals
are:
where i indexes the sensor that receives the corresponding signal.
Coefficients c.sup.0.sub.i,1, c.sup.h.sub.i,1 and c.sup.v.sub.i,1
are the in-phase amplitudes of the received signals. Coefficients
c.sup.0.sub.i,2, c.sup.h.sub.i,2 and c.sup.v.sub.i,2 are the
quadrature amplitudes of the received signals. Because
.omega..sub.1 is sufficiently low that receiver 14 is in the near
fields generated by the loop antennas, in principle the quadrature
amplitudes should be identically zero. Because of inevitable phase
distortions, for example in reception circuitry 34, the quadrature
amplitudes generally are not zero.
Note that amplitudes c.sup.0.sub.i,j, c.sup.h.sub.i,j and
c.sup.v.sub.i,j (j=1,2)could be obtained by using only loop
antennas 26, 28 and 30. The sampled signals obtained by energizing
loop antennas 26, 28 and 30 separately with identical sinusoidal
currents of angular frequency .omega..sub.1 are:
the coefficients c.sup.1.sub.i, c.sup.3.sub.i and c.sup.5.sub.i
being in-phase amplitudes and the coefficients c.sup.2.sub.i,
c.sup.4.sub.i and c.sup.6.sub.i being quadrature amplitudes.
Because the field radiated by loop antennas 26 and 28 when
identical currents J flow therein is the same as the field
generated by loop antenna 48 when current J flows therein,
By definition,
Finally, the fact that the field radiated by loop antenna 48 could
also be emulated by identical currents flowing through loops 30 and
46 gives
In the preferred mode of the operation of the system of the present
invention, loop antennas 26, 28 and 30 are energized simultaneously
with sinusoidal currents of angular frequencies .omega..sub.1,
.omega..sub.2 and .omega..sub.3, respectively. The sampled signals
now are
Note that now, amplitudes c.sub.i1 and c.sub.i2 refer to frequency
.omega..sub.1, amplitudes c.sub.i3 and c.sub.i4 refer to frequency
.omega..sub.2, and amplitudes c.sub.i5 and c.sub.i6 refer to
frequency .omega..sub.3. The sampled signals are organized in a
matrix s of three rows, one row for each sensor of receiver 14, and
as many columns as there are times t.sub.m, one column per time.
Amplitudes c.sub.ij are organized in a matrix c of three rows and
six columns. The matrices s and c are related by a matrix A of six
rows and as many columns as there are in matrix s:
Almost always, there are many more than six columns in matrix s,
making equation (8) highly overdetermined. Because the transmission
frequencies and the reception times are known, matrix A is known.
Equation (8) is solved by right-multiplying both sides by a right
inverse of matrix A: a matrix, denoted as A.sup.-1, such that
AA.sup.-1 =I, where I is the 6.times.6 identity matrix. Right
inverse matrix A.sup.-1 is not unique. A particular right inverse
matrix A.sup.-1 may be selected by criteria that are well known in
the art. For example, A.sup.-1 may be the right inverse of A of
smallest L.sup.2 norm. Alternatively, matrix c is determined as the
generalized inverse of equation (8):
where the superscript "T" means "transpose". The generalized
inverse has the advantage of being an implicit least squares
solution of equation (8).
In the special case of evenly sampled times t.sub.m, solving
equation (8) is mathematically equivalent to the cross-correlation
of WO 96/05768. Equation (8) allows the sampling of the signals
from receiver 14 at irregular times. Furthermore, there is no
particular advantage to using frequencies .omega..sub.1,
.omega..sub.2 and .omega..sub.3 that are integral multiples of a
base frequency. Using closely spaced frequencies has the advantage
of allowing the use of narrow-band filters in reception circuitry
34, at the expense of the duration of the measurement having to be
at least about 2.pi./.DELTA..omega., where .DELTA..omega. is the
smallest frequency spacing, except in the special case of two
signals of the same frequency and different phases.
Because receiver 14 is in the near field of transmitter 24,
coefficients c.sub.ij of equation (7) are the same as coefficients
c.sup.j.sub.i. It follows that equations (1)-(6) still hold, and
either of two 3.times.3 matrices M can be formed from the elements
of matrix c for further processing according to the description in
co-pending Israel Patent Application 122578, an in-phase matrix
##EQU1##
or a quadrature matrix ##EQU2##
Note that because the system of the present invention is a
closed-loop system, there is no sign ambiguity in M, unlike the
corresponding matrix of co-pending Israel Patent Application
122578.
Let T be the orthonormal matrix that defines the rotation of probe
10 relative to the reference frame of transmitter 24. Write M in
the following form:
where T.sub.0 is an orthogonal matrix and E is in general a
nonorthogonal matrix. In general, T.sub.0 and E are functions of
the position of probe 10 relative to the reference frame of
transmitter 24. Let
W.sup.2 is real and symmetric, and so can be written as W.sup.2
=Pd.sup.2 P.sup.T =(PdP.sup.T).sup.2, where d.sup.2 is a diagonal
matrix whose diagonal elements are the (real and positive)
eigenvalues of W.sup.2 and where P is a matrix whose columns are
the corresponding eigenvectors of W.sup.2. Then W=PdP.sup.T =E also
is symmetric. Substituting in equation (12) gives:
so that
If T.sub.0 is known, then T, and hence the orientation of probe 10
with respect to the reference frame of transmitter 24, can be
computed using equation (15).
For any particular configuration of the antennas of transmitter 24,
T.sub.0 may be determined by either of two different calibration
procedures.
In the experimental calibration procedure, probe 10 is oriented so
that T is a unit matrix, probe 10 is moved to a succession of
positions relative to transmitter 24, and M is measured at each
position. The equation
gives T.sub.0 at each of those calibration positions.
There are two variants of the theoretical calibration procedure,
both of which exploit reciprocity to treat receiver 14 as a
transmitter and transmitter 24 as a receiver. The first variant
exploits the principle of reciprocity. The sensor elements are
modeled as point sources, including as many terms in their
multipole expansions as are necessary for accuracy, and their
transmitted magnetic fields in the plane of transmitter 24 are
calculated at a succession of positions relative thereto, also with
probe 10 oriented so that T is a unit matrix. The EMF induced in
the antennas of transmitter 24 by these time-varying magnetic
fields is calculated using Faraday's law. The transfer function of
reception circuitry 34 then is used to compute M at each
calibration position, and equation (16) gives T.sub.0 at each
calibration position. In the second variant, the magnetic field
generated by each antenna of transmitter 24 at the three
frequencies .omega..sub.1, .omega..sub.2 and .omega..sub.3 is
modeled using the Biot-Savart law. Note that each frequency
corresponds to a different sensor 16, 18 or 20. The signal received
at each sensor is proportional to the projection of the magnetic
field on the sensitivity direction of the sensor when object 10 is
oriented so that T is a unit matrix. This gives the corresponding
column of M up to a multiplicative constant and up to a correction
based on the transfer function of reception circuitry 34.
To interpolate T.sub.0 at other positions, a functional expression
for T.sub.0 is fitted to the measured values of T.sub.0.
Preferably, this functional expression is a polynomial. It has been
found most preferable to express the Euler angles .alpha., .beta.
and .gamma. that define T.sub.0 as the following 36-term
polynomials. The arguments of these polynomials are not direct
functions of Cartesian coordinates x, y and z, but are combinations
of certain elements of matrix W that resemble x, y and z,
specifically, a=W.sub.13 /(W.sub.11 +W.sub.33), which resembles x;
b=W.sub.23 /(W.sub.22 +W.sub.33), which resembles y, and
c=log(1/W.sub.33), which resembles z. Using a direct product
notation, the 36-term polynomials can be expressed as:
where AZcoe, ELcoe and RLcoe are 36-component vectors of the
azimuth coefficients, elevation coefficients and roll coefficients
that are fitted to the measured or calculated values of the Euler
angles. Note that to fit these 36-component vectors, the
calibration procedure must be carried out at at least 36
calibration positions. At each calibration position, W is computed
from M using equation (13), and the position-like variables a, b
and c are computed from W as above.
Similarly, the Cartesian coordinates x, y and z of probe 10
relative to the reference frame of transmitter 24 may be expressed
as polynomials. It has been found most preferable to express x, y
and z as the following 36-term polynomials:
where Xcoe, Ycoe and Zcoe are 36-component vectors of the
x-coefficients, the y-coefficients, and the z-coefficients,
respectively; and d=log(c). As in the case of the Euler angles,
these position coordinate coefficients are determined by either
measuring or computing M at at least 36 calibration positions and
fitting the resulting values of a, b and c to the known calibration
values of x, y and z. Equations (17) through (22) may be used
subsequently to infer the Cartesian coordinates and Euler angles of
moving and rotating probe 10 noniteratively from measured values of
M.
Although the antenna configuration illustrated in FIGS. 1 and 6 is
the most preferred configuration, other configurations fall within
the scope of the present invention. FIGS. 7A, 7B and 7C show three
alternative configurations of paired adjacent loop antennas 26' and
28'. The arrows indicate the direction of current flow that
emulates a single loop antenna coincident with the outer perimeter
of antennas 26' and 28'. Other useful coplanar overlapping antenna
configurations are described in PCT Publication No. WO 96/03188,
entitled "Computerized game Board", which is incorporated by
reference for all purposes as if fully set forth herein.
FIG. 8 is a schematic block diagram of driving circuitry 32 for
driving a generic antenna 25 that represents any one of loop
antennas 26, 28 or 30. A digital signal generator 50 generates
samples of a sinusoid that are converted to an analog signal by a
D/A converter 52. This analog signal is amplified by an amplifier
54 and sent to the positive input 60 of a differential amplifier
58. Loop antenna 25 is connected both to the output 64 of
differential amplifier 58 and to the negative input 62 of
differential amplifier 58. Negative input 62 also is grounded via a
resistor 66. The feedback loop thus set up drives antenna 25 at the
frequency of the sinusoid generated by signal generator 50, and
makes antenna 25 appear to be an open circuit at all other
frequencies.
Unlike the circuitry of WO 97/36143, which acts to offset the
influence of one loop antenna on another, the circuitry of FIG. 8
decouples loop antenna 25 from the other loop antennas. The
superiority of the present invention over WO 97/36143 is evident.
Consider, for example, how WO 97/36143 and the present invention
correct for the mutual inductances of loop antenna 26, radiating at
a frequency .omega..sub.1, and loop antenna 30, radiating at a
frequency .omega..sub.2. The goal is to set up the field of
frequency .omega..sub.1 that would be present if only loop antenna
26, and not loop antenna 30, were present, and to set up the field
of frequency .omega..sub.2 that would be present if only loop
antenna 30, and not loop antenna 26, were present. By Faraday's and
Ohm's laws, the time rate of change of the magnetic flux through
loop antenna 26 is proportional to the current through loop antenna
26, and the time rate of change of the magnetic flux through loop
antenna 30 is proportional to the current through loop antenna 30.
In the absence of loop antenna 30, loop antenna 26 sets up a
certain time-varying magnetic flux of frequency .omega..sub.1
across the area that would be bounded by loop antenna 30 if loop
antenna 30 were present. The method of WO 97/36143 forces the time
rate of change of this magnetic flux through loop antenna 30 to be
zero. Because the magnetic flux has no DC component, the magnetic
flux itself through loop antenna 30 therefore also vanishes, which
is contrary to the situation in the absence of loop antenna 30. By
contrast, the present invention makes loop antenna 30 appear to be
an open circuit at frequency .omega..sub.1 and so does not change
the magnetic flux from what it would be in the absence of loop
antenna 30.
FIG. 9 shows, schematically, a C-mount fluoroscope 80 modified
according to the present invention for simultaneous real-time image
acquisition and intrabody navigation. Fluoroscope 80 includes the
conventional components of a C-mount fluoroscope: an x-ray source
82 and an image acquisition module 84 mounted on opposite ends of a
C-mount 78, and a table 86 whereon the patient lies. Image
acquisition module 84 converting x-rays that transit the patient on
table 86 into electronic signals representative of a 2D image of
the patient. C-mount 78 is pivotable about an axis 76 to allow the
imaging of the patient from several angles, thereby allowing the
reconstruction of a 3D image of the patient from successive 2D
images. In addition, either a receiver 114, similar to receiver 14,
or transmitter 24, is rigidly mounted on C-mount 78. Receiver 114
or transmitter 24 serves to define a frame of reference that is
fixed relative to C-mount 78. The other components shown in FIG. 1,
i.e., driving circuitry 32, reception circuitry 34, and
control/processing unit 36, are connected to transmitter 24 and to
receiver 14 in probe 10 as described above in connection with FIG.
1. In addition, signals from receiver 114 that correspond to the
electromagnetic waves generated by transmitter 24' are sent to
reception circuitry 134 that is identical to reception circuitry
34, and controller/processor 36 directs the reception of received
signals by reception circuitry 134 and the acquisition of an image
of the patient by image acquisition module 84 of fluoroscope
80.
By determining the position and orientation of probe 10 relative to
the frame of reference defined by transmitter 24,
controller/processor 36 determines the position and orientation of
probe 10 relative to each acquired 2D image. Alternatively, the
electromagnetic signals are transmitted by a transmitter 24' that
is not attached to C-mount 78, and controller/processor 36
determines the position and orientation of probe 10 relative to the
2D images by determining the positions and orientations of
receivers 14 and 114 relative to transmitter 24'.
Controller/processor 36 synthesizes a combined image that includes
both the 3D image of the patient acquired by fluoroscope 80 and an
icon representing probe 10 positioned and oriented with respect to
the 3D image of the patient in the same way as probe 10 is
positioned and oriented with respect to the interior of the
patient. Controller/processor 36 then displays this combined image
on a monitor 92.
C-mount fluoroscope 80 is illustrative rather than limitative. The
scope of the present invention includes all suitable devices for
acquiring 2D or 3D images of the interior of a patient, in
modalities including CT, MRI and ultrasound in addition to
fluoroscopy.
Under certain circumstances, the image acquisition and the
intrabody navigation may be done sequentially, rather than
simultaneously. This is advantageous if the medical imaging
facilities and the medical treatment facilities can not be kept in
the same location. For example, the human skull is sufficiently
rigid that if a receiver of the present invention is rigidly
mounted on the head of a patient using an appropriate headband,
then the position and orientation of the receiver is a sufficient
accurate representation of the position and orientation of the
patient's head to allow intracranial navigation. FIG. 11 shows a
head 94 of a patient inside a (cut-away) CT scanner 98. As in the
case of fluoroscope 80 of FIG. 9, receiver 114 and transmitter 24
are rigidly attached to CT scanner 98, transmitter 24 being so
attached via an arm 100. CT scanner 98 acquires 2D x-ray images of
successive horizontal slices of head 94. A receiver 214 is rigidly
mounted on head 94 using a headband 96. As the 2D images are
acquired, the position and orientation of receiver 214 with respect
to each image is determined by the methods described above for
determining the position and orientation of probe 10 with respect
to the 2D images acquired by fluoroscope 80. These positions and
orientations are stored, along with the 2D images, in
control/processing unit 36. Subsequently, during medical treatment
of head 94 that requires navigation of probe 10 through head 94,
the position and orientation of probe 10 in head 94 is determined
using signals from receivers 14 and 214 in the manner described
above for positioning and orienting probe 10 with respect to
C-mount 78 of fluoroscope 80 using receivers 14 and 114. Given,
now, for each 2D CT image, the position and orientation of probe 10
with respect to receiver 214 and the position and orientation of
receiver 214 with respect to that 2D image, it is trivial to
determine the position and orientation of probe 10 with respect to
that 2D image. As in the case of the simultaneous imaging and
navigation depicted in FIG. 9, controller/processor 36 now
synthesizes a combined image that includes both the 3D image of
head 94 acquired by CT scanner 98 and an icon representing probe 10
positioned and oriented with respect to the 3D image of head 94 in
the same way as probe 10 is positioned and oriented with respect to
head 94. Controller/processor 36 then displays this combined image
on monitor 92.
As in the case of fluoroscope 80, CT scanner 98 is illustrative
rather than limitative. The scope of the present invention includes
all suitable devices for acquiring 2D or 3D images of a limb of a
patient, in modalities including MRI, ultrasound and fluoroscopy in
addition to CT. Note that this method of image acquisition followed
by intrabody navigation allows the a centrally located imaging
device to serve several medical treatment facilities.
FIG. 14 is a partially exploded, partial perspective view of a
C-mount fluoroscope 80' modified according to one aspect of the
present invention. Like C-mount fluoroscope 80, C-mount fluoroscope
80' includes an x-ray source 84 and an image acquisition module 82
at opposite ends of a C-mount 78. Image acquisition module 82
includes an image intensifier 83, a front face 85 whereof faces
x-ray source 84, and a CCD camera 87, mounted on the end of image
intensifier 83 that is opposite front face 85, for acquiring images
that are intensified by image intensifier 83. Image intensifier 83
is housed in a cylindrical housing 91. In addition, fluoroscope 80'
includes an annular compensator 500 made of a magnetically
permeable material such as mu-metal.
The need for compensator 500 derives from the fact that front face
85 is electrically conductive. The electromagnetic waves generated
by transmitter 24 or 24' induce eddy currents in front face 85 that
distort the electromagnetic field sensed by receiver 14. Placing a
mass of a magnetically permeable substance such as mu-metal in the
proper spatial relationship with front face 85 suppresses this
distortion. This is taught, for example, in U. S. Pat. No.
5,760,335, to Gilboa, which patent is incorporated by reference for
all purposes as if fully set forth herein, in the context of
shielding a CRT from external radiation without perturbing the
electromagnetic field external to the CRT.
Preferably, compensator 500 is a ring, 5 cm in axial length, of mu
metal foil 0.5 mm thick. Compensator 500 is slidably mounted on the
external surface 89 of cylindrical housing 91, as indicated by
double-headed arrows 504, and is held in place by friction. It is
straightforward for one ordinarily skilled in the art to select a
position of compensator 500 on housing 91 that provides the optimal
suppression of distortions of the electromagnetic field outside
image intensifier 83 due to eddy currents in front face 85.
It often is desirable to retrofit a new apparatus such as receiver
14 to an existing catheter rather than to design a new probe 10
that includes both the new apparatus and the functionality of an
already existing probe. This retrofit capability is particularly
important if probe 10 would have been used for medical
applications, and both the apparatus and the existing probe had
already been approved for medical applications by the relevant
regulatory bodies. Such a retrofit capability then would preclude
the need to obtain regulatory approval for the new probe, a process
that often is both expensive and time-consuming.
FIG. 16 illustrates just such a retrofit capability, for adapting a
satellite 550 to a substantially cylindrical catheter 552 for
invasively probing or treating a body cavity such as a chamber of
the heart. Satellite 550 is an instrumentation capsule that may
contain receiver 14 or any other medically useful apparatus. For
example, satellite 550 may contain an apparatus for ablating
cardiac tissue. A catheter such as catheter 552 is introduced to
the body cavity of a patient via the patient's blood vessels, via
an introducer sheath. It is important that the external diameter of
the introducer sheath be minimized, to reduce the risk of bleeding
by the patient. Consequently, the external diameter of catheter 552
also must be minimized, and any scheme for retrofitting satellite
550 to catheter 552 must allow satellite 550 to be introduced into
the introducer sheath along with catheter 552. It is the latter
requirement that generally precludes simply attaching satellite 550
to catheter 552. In addition, if satellite 550 includes receiver
14, with the intention of using receiver 14 to track the position
and orientation of catheter 550, then, when satellite 550 and
catheter 552 are deployed within the body cavity, satellite 550
must have a fixed position and orientation relative to catheter
552.
The retrofitting scheme of FIG. 16 achieves these ends by providing
satellite 550 and catheter 552 with a mechanism for providing only
a loose mechanical connection between satellite 550 and catheter
552 as satellite 550 and catheter 552 are introduced to the body
cavity, and only then securing satellite 550 to catheter 552 at a
fixed position and orientation relative to catheter 552. FIG. 16A
shows a thin flexible tether 554 attached to proximal end 556 of
satellite 550. Tether 554 provides a mechanical link to the outside
of the patient. Depending on the instrumentation installed in
tether 554, tether 554 may also provide a communications link to
the outside of the patient. For example, if satellite 550 includes
receiver 14, then extensions of wire pairs 38 are included in
tether 554. Rigidly attached to tether 554 is a hollow cylindrical
sleeve 558 whose inner diameter is the same as the outer diameter
of catheter 552.
The remainder of the mechanism for reversibly securing satellite
550 to catheter 552 is shown in FIG. 16B. Catheter 552 is provided,
near distal end 564 thereof, with a pocket 560 made of a flexible,
resilient, elastic material. Pocket 560 is attached rigidly to the
outer surface of catheter 552. Pocket 560 includes an aperture 562,
which is adjacent catheter 552 at the proximal end of catheter 552,
and which accommodates tether 554. Pocket 560 is sized to
accommodate satellite 550 snugly therein via an opening in distal
end 566 of pocket 560.
Satellite 550, catheter 552 and the associated securing mechanism
are assembled as shown in FIG. 16C, with tether 554 running through
aperture 562, sleeve 558 encircling catheter 552 proximal of pocket
560, and satellite 550 distal of pocket 560. Catheter 552 and
tether 554 are shown emerging from the distal end of a protective
jacket 568. Preferably, sleeve 558 is made of a low-friction
material such as Teflon.TM., to allow sleeve 558 to slide freely
along catheter 552. The assembly shown in FIG. 16C is introduced to
the introducer sheath with satellite 550 in front of catheter 552.
During this introduction, pocket 560 is compressed against the
outer surface of catheter 552 by the introducer sheath. Tether 554
is sufficiently flexible to bend along with catheter 552 and jacket
568 as the assembly shown in FIG. 16C passes through the patient's
blood vessels, but is sufficiently rigid to push satellite 550
ahead of distal end 564 of catheter 552 as catheter 552 is inserted
into the patient. As a result, satellite 550 and distal end 564 of
catheter 552 reach interior of the targeted body cavity in the
configuration illustrated in FIG. 16C. At this point, pocket 560
opens, and tether 554 is pulled to withdraw satellite 550 into
pocket 560 via the opening in distal end 566 of pocket 560.
Satellite 550 and tether 554 now are held by pocket 560, sleeve 558
and jacket 568 in a fixed position and orientation relative to
catheter 552, as illustrated in FIG. 16D.
Subsequent to treatment, tether 554 is pushed to restore the
configuration shown in FIG. 16C, to allow catheter 552 and
satellite 550 to be withdrawn from the patient.
While the invention has been described with respect to a limited
number of embodiments, it will be appreciated that many variations,
modifications and other applications of the invention may be
made.
* * * * *